The only thing I could find reference I could find re changing a PTA after it's been issued is -
Modifying a PTA
If you require modification to a PTA (i.e. you have changed your mind about the weapon to be acquired) you must return the original PTA to Weapons Licensing for modification.Now, while this doesn't specifically mention changing the dealer, this may be broadly interpreted to cover all modifications of a PTA. NSW has a similar rule.
This no doubt may crop up - Dealer has lost the agency and can no longer get the firearm, Model has been discontinued/ out of stock but another dealer has it, Dealer has ceased business.
How did you get on?